It is well known that contact lenses are becoming more and more popular in our society. Many people are wearing contact lenses as opposed to conventional eyeglasses for reasons of convenience, improved appearance, lighter weight, and correction of sight abnormalities over a broad visual field. Most conventional contact lenses are made from methyl methacrylate. Lenses made from this material are known as "hard lenses". These lenses suffer from many deficiencies. For example, such lenses frequently produce corneal edema and/or a condition of extreme discomfort to the wearer's eye after repeated periods of extended wear, i.e., eighteen hours or more. This situation is known to be due to "oxygen starvation" and may also be associated with inadequate dissipation of carbon dioxide.
The epithelium of the cornea requires oxygen which is usually supplied from the oxygen dissolved in tears. However, because of the manner in which lenses conform to the contour of the eye, the flow of lacrimal fluid is greatly curtailed beneath the lens. This reduction in fresh lacrimal fluid is not desirable as it substantially reduces the contact of the eye with oxygen. Therefore, it is extremely important that the the lens material itself be gas permeable. Prior art lenses have been of a material and thickness which fails to admit sufficient oxygen and/or release sufficient carbon dioxide to maintain a healthy normal condition for the eye tissue and cornea covered, especially when the lens is worn continuously for extended periods of time. In other words, the conventional lens cannot breathe through the body of the lens satisfactorily.
Due to the above problems, many workers in the field have experimented with the production of soft contact lenses. The presently known soft lenses are made of hydrophilic polymers, mainly comprising polyhydroxethyl methacrylate (known in the art as "HEMA"). These hydrogel soft lenses are an improvement over the hard lenses but the materials themselves are not gas permeable. However, these materials absorb water and swell until equilibrium is attained and therefore possess a high degree of hydration which is directly related to the mode of oxygen transport. The highly hydrated lenses are able to obtain satisfactory oxygen transport levels but suffer from several resulting problems. First, since the soft lenses are used in the swollen state, the molecular materials of their composition are markedly reduced in mechanical strength and are extremely fragile. Due to this fragileness, the thickness of the lens must be increased and therefore these prior art soft lenses are ill-suited for the preparation of ultra-thin corneal lenses. By increasing the thickness of the lens, the gas permeability of the lens is thereby decreased forming a vicious cycle between gas permeability and strength.
In making an ultra-thin lens, the greater the strength and the greater the refractive index of the material used, the better the resulting thin lens.
A second problem associated with the prior art soft lenses is that since they are always worn in the wet and swollen state, they are easily contaminated with bacteria. Therefore, they need to be sterilized once a day by boiling. This boiling treatment is not only troublesome, but often causes decomposition and breakdown of the lens material. Thus, the prior art soft lenses are very short lived.
The disclosed invention obviates the above deficiencies in the prior art by providing a copolymer suitable for producing contact lenses which have a superiorly high strength and refractive index, that can withstand sterilization, and, in addition, offer superior gas permeability. These properties make the fabrication of an ultra-thin lens a practical reality.